Generally, in an injection device of an injection molding machine, a screw rotating motor is connected to the rear end of a screw inserted into a heating cylinder to rotate the screw about an axis in the heating cylinder, as shown in FIG. 1. Then, the screw rotating motor is disposed such that the rotary shaft of the rotating rotor thereof projects forward (not shown) and connected to the screw through a coupling, and the like. Further, the injection device includes a screw moving means (refer to reference numeral 4 of FIG. 1) to movably support the screw axially rearward and forward. A servo motor using a permanent magnet is ordinarily employed as the screw moving means to controllably drive the movement of the screw in an axial direction.
A molding material such as a resin, and the like, which has been fed into the heating cylinder by rotating the screw about the axis by driving the screw rotating motor, is heated, melted, and plasticized while being kneaded for a predetermined period of time, and a predetermined amount of the molding material is stored forward of the heating cylinder (when it is metered). In the metering, a back pressure acting to press the screw axially backward is produced by the reaction for pushing out the molding material forward of the heating cylinder, and the screw supported by the screw moving means is moved axially backward according to the back pressure. The molding material is plasticized for a predetermined period of time by moving the screw axially rearward by a predetermined stroke while properly rotating it, thereby an accurately metering amount of the molding material is stored forward of the heating cylinder.
Thereafter, a nozzle disposed at the extreme end of the heating cylinder is caused to be in nozzle-touch with a sprue of a clamped metal mold by moving the injection device forward, and the screw is moved forward by driving the screw moving means to thereby inject the molding material having been plasticized in the heating cylinder and stored forward thereof in the proper amount from the nozzle with a predetermined pressure (filling pressure) so as to fill the metal mold with the molding material, and thereafter the molding material is held with a predetermined pressure (holding pressure, in injection). The screw is subjected to the reaction (injection pressure) for pushing out the molding material from the nozzle also in the injection when it is moved forward in the heating cylinder by driving the screw moving means. The fill pressure and the holding pressure, that is, the injection pressure received by the screw in the injection, acts axially rearward of the screw, similarly to the back pressure in the metering. The holding pressure is ordinarily set lower than the fill pressure. Note that, when the back pressure is not particularly distinguished from injection pressure in the following description, a force, which acts on the screw so as to push it axially rearward by the reaction of the molding material in the metering and in the injection, is referred to as a molding material pressure.
Japanese Examined Patent Application Publication No. 63-25934 discloses a method of and an apparatus for controlling the back pressure of a screw of an injection device. The screw back pressure control method has such a feature that the rearward movement of a screw that is caused when a molding material is charged by the rotation of the screw is converted into a rotating motion so as to control the rotating force of the screw. Further, the screw back pressure control apparatus disclosed in the publication is composed of an extending shaft connected to the rear end portion of the screw integrally therewith and having a gear for applying a rotating force to the screw with the shaft end thereof rotatably coupled with a screw hold member, the screw hold member disposed to a pair of support shaft disposed in a housing so as to move forward and rearward, a screw shaft for converting the rearward movement of the screw that moves together with the screw hold member into a rotating motion, and a brake unit connecting to the shaft portion of the screw shaft as well as controlling the rotating force of the shaft portion caused by the movement of the screw hold member. That is, an object of the technology disclosed in the publication for converting the rearward movement of the screw, which is caused following to the charge of a molding material charged by the rotation of the screw about an axis, into a rotating motion and for controlling the running torque of the rotating motion is to provide a method of and an apparatus capable of controlling a screw back pressure without using a hydraulic pressure. Then, when injection is executed, the screw hold member is moved forward by rotating the screw shaft. Note that, the embodiment describes a case in which a hysteresis brake is used as the brake unit. Further, the embodiment describes that the shafts such as the screw and the screw shaft are connected to a servo motor that acts also as a rotating motor for applying a rotating force to the shafts through a gear and a drive belt, although this is an ordinary arrangement in conventional technologies.
Further, to control the molding material pressure of the screw, a molding material pressure detecting means such as a load cell, and the like is mounted on a member receiving a load due to the rearward movement of the screw, the molding material pressure is recognized as an absolute value in response to a signal output from the molding material pressure detecting means, and the axial movement of the screw is controlled based on the absolute value.
That is, the electric output signal output from the load cell is recognized as an absolute value showing a back pressure in metering and further recognized as an absolute value showing an injection pressure in injection as described above, and the axial pressures of the screw in the metering and the injection are controlled based on these absolute values, respectively. The electric signal output from the load cell is ordinarily amplified by an amplifier and passed through a low-pass filter for eliminating the electric disturbance thereof such as noise, and the like.
Note that since the back pressure is about 15% or less of the fill pressure of the injection pressure (ordinarily 1/10 or less) and controlled in a very small value, the electric signal detecting the signal is liable to be influenced by the electric disturbance thereof. The holding pressure of the injection pressure may be as large as the back pressure. Further, when the moving position of the screw is controlled, greater emphasis is placed on control responsiveness in injection than in metering because an injection time is much shorter than a metering time. Further, when an electric signal is effectively used as a feed-back signal after noise is eliminated therefrom, the frequency band in which the feed-back signal can be used is different between a back pressure and an injection pressure produced in the screw.
Therefore, in general, the constant of a low-pass filter for eliminating the electric disturbance is set to a given value so as to provide a relatively high frequency band by placing emphasis on the control responsiveness in the injection, and the signal output from the low-pass filter is captured as a feed-back signal for controlling the axial movement of the screw in a frequency band in which the electric disturbance is eliminated by the given constant.
However, the technology disclosed in Japanese Examined Patent Application Publication No. 63-25934 among the above conventional technologies converts the rearward movement of the screw caused by the back pressure only in the metering into the rotating motion. Accordingly, a problem arises in that a small force such as the back pressure caused by the rearward movement of the screw in the metering cannot be effectively converted, the back pressure to be controlled is dispersed, and thus the back pressure in the metering cannot be accurately controlled. This is because the efficiency of the screw shaft for converting the rearward movement into the rotating motion is bad.
Further, in this technology, a problem arises in that the control responsiveness cannot be improved and the back pressure cannot be accurately controlled in the metering. This is because the position of the screw in the metering cannot help being controlled, so to speak, only indirectly by controlling the rotating force converted from the rearward movement of the screw simply by a brake.
Further, in the technology, in which the load cell is mounted on the member receiving the load applied by the rearward movement of the screw to detect the molding material pressure produced to the screw, of the conventional technologies described above, when the electric signal, from which the electric disturbance such as noise, and the like has been eliminated by the given constant, is captured as the feedback signal in the injection and metering, a problem arises in that the electric signal cannot be used as an effective control signal for controlling the axial movement of the screw in the injection and metering. This is because the magnitude, control responsiveness, and frequency band of the output electric signal are not proper in the injection and the metering.
Further, in this technology, the linearity between the received back pressure and the output signal of the load cell is bad and thus it is difficult to accurately control the back pressure of the screw. This is because that since the back pressure is ordinarily much smaller than the injection pressure as described above, the low pressure detecting region of the load cell is used.
Furthermore, in the technology in which the permanent magnet is used to drive the servo motor for controlling the axial movement of the screw, a problem arises in that the axial movement of the screw cannot be accurately controlled. This is because a cogging torque produced by the permanent magnet of the servo motor makes the torque of the screw shaft unstable and a pulsation is produced thereby. The influence caused by the above problems becomes serious particularly when the relatively small back pressure in the metering and the holding pressure after injection and filling of the molding material pressure are controlled.
In addition to the above problems, a problem is also arisen in that the dimension of the injection device is increased in the axial direction and a relatively large installation space is necessary, thereby the injection device cannot be made compact. This is because the rotating shaft of the rotating rotor of the screw rotating motor projects forward as described above. Then, there is a possibility that a problem is arisen in quietness in operation due to friction noise and gear mesh noise. This is because the gear and the drive belt are interposed between the screw rotating motor and the screw. Further, a problem is arisen in that it is difficult to improve controllability because there is a possibility that an error is caused in the rotation control of the screw by an increase in backlash, and the like due to wear.
An object of the present invention, which was made in view of the aforementioned problems, is to provide an injection molding machine and a method of controlling the injection molding machine capable of controlling the axial pressure of a screw by accurately detecting a molding material pressure received by the screw in the state in which bearings and ball screws of the injection molding machine are not influenced by wear, and the like. Further, an object of the present invention is to provide an injection molding machine that is compact in its entirety and excellent in the rotation controllability and the quietness of a screw.